Separate FieldParser to its own file

So far, FieldParser was an implementation detail in csv_reader.cc.

With the new design for deserialising passwords from CSV (see
https://docs.google.com/document/d/1wsZBl93S_WGaXZqrqq5SP08LVZ0zDKf6e9nlptyl9AY/edit?usp=sharing),
it will be needed from the new parser class as well. Moreover, having
it in a separate file with a public header makes testing easier.

This CL also renames the class by adding the prefix "CSV". This was
not necessary while inside csv_reader.cc, but it seems an appropriate
context for a self-standing class, in particular given that there is a
whole different group of code focusing on parsing forms from HTML
elements.

Bug: 934326
Change-Id: I2ec5516771b1a6f6ef67870ff51b0db9ffa0b154
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660659
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Auto-Submit: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669726}
7 files changed